In the prior art, semiconductor wafers, particularly silicon wafers, have been polished by silica based slurries using many different types of polishing machines. Although a number of improvements have been made to the silica based slurry, there has been no substitute to colloidal silicon dioxide for achieving an acceptable degree of perfection.
A typical abrasive slurry for polishing silicon wafers is taught in J. S. Basi U.S. Pat. No. 4,057,939 assigned to the same assignee as this invention. The slurry comprises colloidal SiO.sub.2 mixed with NaCO.sub.3 and an oxidizing agent such as sodium dichloroisocyanurate. It is believed that the polishing mechanism is due to the rapid oxidation of silicon to SiO.sub.2 by the polishing slurry component, followed by softening of SiO.sub.2 thus formed to give silica gel (Si(OH).sub.4) which, in turn, further absorbs the slurry components and becomes softer and is easily removed by the scrubbing action of the polishing slurry. Thus, the polishing mechanism is chemical-mechanical.
Polishing may be effected in machines such as described in Goetz et al. U.S. Pat. No. 3,691,694 including modifications thereof in accordance with the planetary configuration shown in IBM Technical Disclosure Bulletin, pp. 1760-1761, Vol. 15, No. 6, November 1972.
It is also known that montmorillonite minerals have been mined in a large volume and have been used as a binder for sand foundry molds and as a thickener for oil-well drilling muds. "The Colloid Chemistry of Silica and Silicates" by R. G. Iler, pp. 190-199, Cornell University Press, 1955 describes the chemical structure and nature of montmorillonite minerals.
G. G. Rayner, U.S. Pat. No. 3,966,432 entitled "Liquid Abrasive Compositions Containing A Silicate Suspending Agent" teaches that natural and synthetic clays including hydrophobically treated montmorillonite can be used as a suspending agent for abrasive material for cleaning hard surfaces of such materials as stainless steel, ceramics and vitreous enamel. The abrasive materials listed are calcite (ground marble), silica, felspar, pumice, hieselguhr and labradorite.
C. W. Dahlin, U.S. Pat. No. 4,169,796 teaches that sodium montmorillonite clay when it is mixed with dicalcium phosphate and distilled demineralized water creates an effective and useful shaving lotion and a product for polishing and cleaning dentures.
Nonetheless, there has been no attempt in the known prior art to utilize montmorillonite clay in the manufacture of semiconductor devices, particularly for polishing semiconductor wafers.